Capacity control of refrigeration system



' July 19, 1960 EXPANSION c. BOLING CAPACITY CONTROL OF REFRIGERATION SYSTEM Filed Dec. 20, 1955 20 VALVE [8 WATER OHILLER [3 ATER WATER ZI I OUTLET LET SIGHT GLASS SUCTION LINE R-IN

RECEIVER CONDENSER 58 fillma @mum COMPRESSOR z z y I I a 55 g 5 7 a 59 5 HOLD BACK 5 5 VALVE 4 VALVE 6/ \f lllll liil 6O INVENTQR Cec o' l ,Boiny Ail/1764', m

I I I Al IORN CAPACITY CONTROL OF REFRIGERATION SYSTEM Cecil Boling, West Hartford, Court, assignor to Heat-X, Inc., Brewster, N.Y.

This invention relates to refrigeration, and more in particular to improving the operation of refrigeration systems. One phase of the invention provides for more efficient use of the evaporator of refrigeration systems, with reliable control of the operation at all times. Another phase. of the invention provides for automatic control of the capacity of refrigeration systems whereby the refrigeration system varies its capacity automatically to accommodate itself to the load demands placed upon it.

An object of this invention is to provide for the improved operation of refrigeration systems, particularly for cooling water and other liquids. Another object is to provide for improved operation of various types of refrigeration systems where it is important to provide stable or constant evaporator temperatures. A further object is to provide increased capacity in refrigeration systems. A further object is to provide for wide variations in the capacity of refrigeration systems. A further object is to provide for the above while avoiding the difiiculties which have been encountered in the past. A still further object IisIto provide structure which accomplishes the above and yet which is adaptable to many conditions of use and is practical in every respect. These and other objects will be in part obvious, and in part pointed out below.

In the drawing, the single figure is a schematic representation of one embodiment of the invention with parts of certain of the elements broken away to show the inlternal structure.

The illustrative embodiment of the present invention is a water chiller of the packaged type. In the schematic representation, a compressor 2 delivers hot, compressed refrigerant gas through a line 4 to a Water-cooled condenser 6 where the refrigerant is condensed. Condenser 6 is of the'shell and tube type, with the refrigerant flowing through tubes from a header system at the left to a header system at the right. Cooling water enters at the right through a line 5, and flows fromthe unit at the left through a line 7. The liquid refrigerant flows through 'a line 8 to a receiver 10 from which it flows through a line 12, a heat interchange unit 14 and a line 16, to an expansion valve 18, and thence to the evaporator circuit of a water chiller unit 24 Line 16 has a refrigerant dryer 22 and a sight glass 24 therein. Water chiller unit 20 is of the shell and tube type with refrigerant headers 13 and 15, between which the refrigerant tubes 17 extend.

' Water to be cooled enters the shell at 19 and flows to the 'left around the tubes 17, and flows from the shell at 21.

The refrigerant evaporates in the evaporator section of chiller 20 and returns to the compressor through a line 26, heat interchange unit 14, and a line 28.

The left-hand section A of unit 14 incorporates the invention and the heat interchange structure disclosed in the co-pending application, Serial No. 402,447, filed January 6, 1954, now Patent No. 2,797,554, and reference may be had thereto for a more detailed discussion of that structure and its operation. However, unit 14 includes additional structure in the form of a right-hand section B, and the unit performs additional functions which will be 2,945,355 Patented July 19', 1960 ice described fully herein. Unit 14 is formed by a cylindrical shell 30 having four tube sheets or header plates 32, 34, 36 and 38 mounted therein which form a refrigerant inlet header 40 at the right, a central header 42 and a refrigerant outlet header 44 at the left. Mounted between tube sheets 32 and 34 is a set of tubes 46 and, similarly, mounted between tube sheets 36 and 38 is a set of tubes 48. Tubes 46 and 48 have reduced ends which extend through the tube sheets and open into the respective headers.

The refrigerant flowing from the evaporator through line 26 in gas or vapor form enters header 40 and flows through tubes 46 to header 42. It then flows through tubes 48 to header 44, from which it flows into tube 28. The liquid refrigerant from line 12 enters the space around tubes 48 between headers 38 and 36 and flows to the right and, at tube sheet 36, the liquid refrigerant flows through a tube 49 around header 42 and back into the shell at the right of tube sheet 34. The liquid refrigerant then flows through the space on the outside of the tubes 46 and, as indicated above, leaves the unit through line 16. Hence, during operation, the refrigerant in gas or vapor form returning from the evaporator flows countercurrent to the liquid refrigerant flowing to the evaporator, and good heat interchange relationships are provided within the shell 30 by the two sets of tubes.

Mounted in a well extending into header 42, is the bulb 50 of the expansion valve 18. Also connected into header 42 is an equalizer line 52 which has its other end connected to the expansion valve, and which equalizes the pressures to compensate for pressure drop due to the refrigerant flow. Connected between the hot gas discharge line 4 and line 26 is a bypass line 54 Which has a holdback valve 56 therein. Line 54 also has a manual shutoff valve 58 therein which is normally open.

Valve 56 has a valve member 53 which is supported from 'a bellows 59 through the valve stem. When the valve is closed, valve 53 is held against the valve seat 55 by the action of the. compressed refrigerant in the valve chamber 57 and the difierential in the pressures on the valve member. The valve may be opened by the expansion of the bellows and the movement of the valve member away from its seat. A spring 61 within the bellows urges the valve member toward the open valve position and, as indicated above, during normal operation the pressures are such as to overcome the force of this spring. However, the force exerted by this spring is adjustable by a knob 60, and the adjustment is such that the valve is closed when the pressure in line 26 is above a predetermined value. At any time that the pressure in line 26 drops below that predetermined value, the combined action of spring 51 and the pressures become unbalanced so that valve member 53 starts to move away from its seat 55. This permits hot refrigerant gas to bleed or flow from line 4 through the bypass line 54 and valve 56 into'line 26, thus to mix with the refrigerant vapor or gas returning from the evaporator toward unit 14. As will be pointed out more fully below, this provides a very satisfactory automatic control upon the capacity of the refrigeration system.

As indicated above, the left-hand section A of the heat interchange unit 14 heats the refrigerant gas or vapor returning from the evaporator to the compressor, and it cools the liquid refrigerant flowing to the evaporator. Simultaneously, the right-hand section B of unit 14 provides subcooling for the liquid refrigerant, and it heats the saturated vapor flowing from the evaporator section of unit 20. Thus, section B of unit 14 acts as a portion of the evaporator. This manner of operation permits the entire evaporator section of unit 20 to be fully flooded with liquid refrigerant at all times so that the entire unit is utilized to perform its cooling function.

This mode of operation is obtained by positioning the sensing bulb in the central header 42 of unit 14 so that the flow of refrigerant to the evaporator is controlled in accordance with the temperature in this header. With prior similar systems, the sensing bulb has been placed at or near the outlet from the evaporator, and the expansion valve has been set to provide a predetermined number of degrees rise from the valve to thesensing bulb. With this arrangement, a portion of the evaporator is relegated to superheating the refrigerant and the evaporator is supplied with refrigerant only in sufficient quantity to fill the remaining portion of the evaporator space. With the arrangement herein disclosed, section B of unit 14 is so constructed, and is of such size, that it provides adequate superheating for the refrigerant. Hence, no portion of the evaporator section of unit 29 is used for superheating refrigerant, and the entire evaporator space is filled with liquid refrigerant.

During operation, saturated vapor leaving unit 20 is heated in section B of unit 14, and it then contacts bulb 50, the temperature of which determines whether or not valve 18 is to be opened or closed. If the temperature of bulb 50 is below its predetermined temperature, there is excessive refrigerant being supplied to the evaporator because insuflicient superheating is being accomplished, and the contrary is true. Hence, for any set of operating conditions within the limits of the capacity of the system, an equilibrium condition will be reached at which the exact amount of liquid refrigerant is supplied to the evaporator to handle the cooling load. If this load increases, the temperature of the refrigerant at bulb t will increase, with the result that valve 18 will be opened further, and if the load decreases, the temperature of bulb 56 will be decreased, so that valve 18 is moved toward its closed position. In either case, a new condition of equilibrium is reached shortly. Under all conditions of operation, the superheated refrigerant flowing fro-m header 42 is further heated in section A of unit 14- prior ,to passage to the compressor.

It has been indicated above, that the bypass line 54 and valve 56 provide capacity control for the system. If the cooling load on unit 20 decreases, there is a tendency for the evaporator temperature and pressure to drop. With prior systems, this has caused difiiculties because a continued drop in evaporator temperature and pressure results in freezing the water in the chiller unit. If such a system were provided with an automatic low'pressure cutfreezing temperature of water causes valve 56 to open.

This permits hot refrigerant gas to flow through line 54 to line 26, and thus mix with the refrigerant vapor flowing from the evaporator toward unit 14. 'The initial elfect of introducing a stream of hot refrigerant gas from the compressor into this cold stream of gasis tocause an increase in the temperature of the gas passing through header 42. This increases the temperature of bulb 5t), and causes valve 18 to be opened further so that more liquid refrigerant is passed to the evaporator section of unit 20. It has been indicated above that the system normally operates with the evaporator section of unit 20 fully flooded with liquid refrigerant. Therefore, the passing of an increased supply of liquid refrigerant through valve 18 causes some liquid refrigerant to pass from the evaporator section with the refrigerant vapor through line 26 toward unit 14. This liquid refrigerant encounters and mixes with the hot compressed gas from line 54, and the to flow through the lower resistance circuit.

mixture passes on to unit 14. The hot refrigerant is effective to evaporate the liquid refrigerant, and the temperature of the mixture is thereby reduced. Hence, the temperature of the bulb 50 isreduced toward its predetermined temperature, and a new equilibrium condition is soon reached. At this time, liquid refrigerant is supplied to unit 249 in an amount exceeding that required to handle the cooling load, and a control stream of hot co pressed gas flows through the bypass line 54 and valve 56 to line 26. With this condition of operation, the temperature of the refrigerant in header 42 is substantially the same as with normal full-load operation and, therefore, the refrigerant returning to the compressor is at substantially the same temperature at all times. it has been found that the system of the illustrative embodiment will operate very satisfactorily at capacities down to 25 percent without short cycling or other difficulties.

It should be noted that an important aspect of the invention is that of supplying some liquid refrigerant to the stream of hot gas which is bypassed from the compres- S01; If this hot gas were mixed with a stream of superheated refrigerant gas, the resulting mixture would still a have an excessively high temperature. With such an arrangement, the returning of the hot mixture would be damaging to the system. If an attempt is made to control the capacity of a refrigeration system by providing a back pressure valve at the outlet of the evaporator, then excessively low pressures will be encountered at the inlet to the compressor. In other words, if the evaporator is provided with a valve which restricts the refrigerant flow when the evaporator pressure tends to drop, then excessively low pressures will be created in the suction line to the compressor. This causes difficulties, including that of maintaining a proper supply of oil in the compressor, and this may cause damage to the apparatus.

Certain aspects of the invention have wide application in the refrigeration field. For example, in test chambers and elsewhere where it is desirable to maintain very accurate control of the relative humidity, a small variation in the evaporator temperature is apt to cause considerable difficulty. With the present invention, the evaporator temperature may be controlled very accurately over wide ranges of operating conditions, and this permits very accurate control of the cooling and resultant effects.

In the illustrative system, the evaporator of unit 20 is formed by a plurality of annular passageways, each formed between a pair of concentrically positioned tubes, and there is a fin assembly under radial compression within the passageway. The construction is similar to that of the unit 14 as discussed above, and as disclosed in the co-pending application identified above.

With the present invention, the liquid refrigerant is subcooled and all of the superheating of the vapor and gas is accomplished outside of the evaporator. These effects combine to reduce the formation of flash gas in the evaporator and to reduce the pressure drop therethrough. Also, the over-all inside coefliicient of heat transfer is raised because all of the internal evaporator surfaces are wetted with refrigerant. With prior systems of the type having the evaporator formed by a number of different circuits which'do not have low resistance to refrigerant how, there has been a tendency for excessive refrigerant 7 If this or another factor causes liquid refrigerant to reach the controlbulb, the refrigerant supply is reduced even though some of the circuits are receiving insufficient refrigerant. With the present invention, this difficulty is avoided.

It will be seen that the objects above set forth have been obtained, .and that the invention represents a substantialadvance in the refrigeration field. The construction and positioning of the heat interchange unit 14 and expansion valve 18, and the location of the sensing bulb :50 provide and maintain liquid refrigerant throughout the evaporator at 'alltimes. The heat interchange unit and the bypass arrangement maintain the compressor operating, even though the load is substantially rean mate du'c'ed. At the same time, the liquid refrigerant is precooled before reaching the evaporator, and all liquid refrigerant is evaporated before it returns to the compressor. When the system is operating at a substantially reduced load, with hot gas being bled into the refrigerant return line, a rise in the suction line pressure causes the bypass valve to start closing and, as the load increases, the valve is completely closed. Short cycling is completely eliminated, and yet the efliciency during normal loads is extremely high.

As many possible embodiments may be made of the mechanical features of the above invention herein described, all without departing. from the scope of the invention, it isto be understood that all matter hereinabove set forth, or shown in the accompanying drawing sense.

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1. In a r igeration systemhaving a compressor, a condenser, an expansion valve and an evaporator interconnected for the flow of refrigerant in a refrigeration cycle, the combinatiop withsaid system of a heat interchange unit having a liquid path along which liquid refrigerant flows from the condenser to the evaporator in heat interchange with a gas and vapor refrigerant path along which refrigerant ilows from the evaporator to the compressor, said heatinterchange .unit having an intermediate sensing zo e through which refrigerant flows from the evaporator and which is not in heat-exchange relationshipwith the liquid refrigerant flowing to the evaporator, a thermostat positioned in said sensing zone of said heat interchange .unit in direct heat exchange relationship with the stream of refrigerant flowing from the evaporator after the evaporation thereof during flow through .a portion of said heat interchange unit, said thermostat being effective to actuate the expansion valve to maintain a supply of liquid refrigerant throughout the entire evaporator, and means to further heat the refrigerant after'its temperature is sensed by said thermostat.

2. In a refrigeration system in accordance with claim 1 in which said thermostat is constructed and arranged to close the expansion valve when liquid refrigerant overflows from the evaporator and .to open the valve when vapor from the evaporator is superheated in .excess of a predetermined amount.

3. In a refrigeration system in accordance with claim 1 wherein said means to further heat the refrigerant includes additional heat interchange means for further heating the refrigerant flowing toward the compressor after it has been sensed by the thermostat.

4. In a refrigeration system having a compressor, a condenser, an expansion valve and an evaporator interconnected for the flow of refrigerant in a refrigeration cycle, the combination with said system of means for maintaining a load on the compressor when the load on the evaporator decreases comprising a chamber in a path through which refrigerant flows from the evaporator to the compressor, a bypass connection between the outlet from the compressor and the inlet to said chamber for delivering hot refrigerant vapor from the compressor to said chamber, and a valve in said bypass connection which is responsive to a condition affected by the load on the evaporator and which is adapted to move gradually from its fully closed position to open said bypass and deliver a controlled stream of hot refrigerant vapor to the chamber for mixture with cold vapor from the evaporator when the load on the evaporator decreases and to move to its fully closed position when .the load on the evaporator increases.

5. A refrigeration system in accordance with claim 4 which includes a thermostat operating said expansion unit having a path through which'hot liquid refrigerant flows from the condenser to the evaporator in heat exchange with a path through which refrigerant flows from the evaporator toward the compressor. I V

7. In a refrigeration system in accordance with claim 4 in which said chamber is formed by a heat interchange unit having two separate sections .connected' in series through which hot liquid refrigerant flows from the compressor toward the evaporator and providing a space between the sections in the path through which refrigerant flows from the evaporator toward the compressor, and in which the expansion valve is actuated by a thermostat having a sensing element in said space which tends to open the expansion valve upon an increase in temperature in said space and tends to close the expansion valve upon a decrease in temperature in said space.

8. In a system for chilling water or the like, a shell andftube evaporator having evaporator space formed within the tubes and liquid cooling space around the tubes and within the shell, refrigerant condensing means which supplies liquid refrigerant to said evaporator space and withdraws vapor refrigerant therefrom, a refrigerant control valve positioned to control the supplying of liquid refrigerant to said evaporator space and having a control element positioned in the returning path of flow of refrigerant from said evaporator space to be responsive valve responsive to the temperature of the mixed vapor solely to a condition of the refrigerant in said'retuming path, and heat interchange means having one flow path through which refrigerant vapor flows from said evaporator space prior to contacting said control element and a liquid flow path through which liquid flows to said valve whereby the liquid refrigerant is subcooled by passing in heat interchange relationship with refrigerant flowing from said evaporator space, said heat interchange means including means along the path of the refrigerant flowing to the compressor to heat the refrigerant after it has contacted said control element, said control valve being set and the elements being so proportioned and arranged that said evaporator space is filled with liquid refrigerant at all times and the refrigerant is superheated a predetermined amount in said heat interchange means.

9. Apparatus as described in claim 8, which includes means responsive to an excessive drop in the pressure along said refrigerant return path to supply hot fluid thereto whereby the first-mentioned heat interchange unit acts to mix the hot fluid with refrigerant flowing from said evaporator space.

10. A system as described in claim 9, wherein said refrigerant condensing means comprises a motor-driven compressor and condenser and includes a receiver for liquid refrigerant, and wherein said hot fluid is compressed refrigerant flowing from said compressor.

11. In the art of refrigeration, the steps of, supplying liquid refrigerant to an evaporating zone, withdrawing refrigerant therefrom along a return path, passing the liquid refrigerant flowing to-said evaporating zone into heat interchange relationship with vapor refrigerant flowing from said evaporating zone in separate heat interchange zones thereby to precool the liquid refrigerant and to superheat the vapor refrigerant, and controlling the amount of liquid refrigerant flowing to said evaporating zone solely in accordance with the temperature of the refrigerant at a point between the heat interchange zones which is subjected to the influence of the overflow of liquid refrigerant from said evaporating zone to maintain said evaporating zone substantially filled with liquid refrigerant at all times.

12. The art as described in claim 11 which includes adding a stream of hot refrigerant vapor to the refrigerant flowing from said evaporating zone in response to a drop in pressure in the evaporating zone to a predetermined value thereby to reduce the influence of the overflow liquid refrigerant.

13. In a refrigeration system having a compressor, a

condenser, an expansion valve and an evaporator interconnected for theflowf'of refrigerant in a refrigeration iiyclefthe' combination with said system of aheat interchange unit having a liquid path along which liquid refrigerant flows from the condenser to the evaporator ii1' heat interchange with a gas and vapor refrigerant path along which refrigerant flows from the evaporator to'the compressor, said heat interchange unit comprising two sections connected in series with a chamber between said petitions in said refrigerant path from the evaporator, and a" thermostat having a sensing element located in said chamber responsive to the temperature of the refrigerant flowing from the evaporator after passage through at least meant said'heat interchange unit for actuating the expansion valve to maintain a supply of liquid refrigerant throughout the entire evaporator.

a refrigeration system having a compressor, a condenser, an expansion valve and an evaporator 'in'terc nnected for the new of refrigerant in a refrigeration cycle, the combination with said system of a heat eicchangeunit having a path for liquid refrigerant flowing froinsaid condenserto said evaporator arranged in heat exchange relationship with a path for refrigerant vapor flowing fromsaid evaporator to said compressor, a therirnos tat for actuating s'aid expansion valve, saidthermos't'at b uig located to sense only the temperature of refrigerant "vpor'flowing from said evaporator after passage through a pojrtion "of the heat exchange unit in heat exchange relation with the liquid refrigerant flowing to said evaporatorand operating said expansionvalve in response to a decrease in temperature when liquid refrigerant overflows fromsaid evaporator, a bypass connection between the outlet from said compressor and the'outlet from said evaporator to supply hot refrigerant vapor from said compressor for mixture with cold refrigerant from said evaporator prior to flow through said heat exchange unit to said thermostatand a valve in said bypass connection which moves in a controlled manner inresponse to a condition affected by the load on said evaporator to open said bypass connection as the load decreases and to close said bypass connection as the load increases.

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